WebSocket

WebSocket is a computer communications protocol, providing full-duplex communication channels over a single TCP connection. The WebSocket protocol was standardized by the IETF as RFC 6455 in 2011, and the WebSocket API in Web IDL is being standardized by the W3C.

WebSocket is designed to be implemented in web browsers and web servers, but it can be used by any client or server application. The WebSocket Protocol is an independent TCP-based protocol. Its only relationship to HTTP is that its handshake is interpreted by HTTP servers as an Upgrade request.^[1] The WebSocket protocol enables interaction between a browser and a web server with lower overheads, facilitating real-time data transfer from and to the server. This is made possible by providing a standardized way for the server to send content to the browser without being solicited by the client, and allowing for messages to be passed back and forth while keeping the connection open. In this way, a two-way (bi-directional) ongoing conversation can take place between a browser and the server. The communications are done over TCP port number 80 (or 443 in the case of TLS-encrypted connections), which is of benefit for those environments which block non-web Internet connections using a firewall. Similar two-way browser-server communications have been achieved in non-standardized ways using stopgap technologies such as Comet.

The WebSocket protocol is currently supported in most major browsers including Google Chrome, Microsoft Edge, Internet Explorer, Firefox, Safari and Opera. WebSocket also requires web applications on the server to support it.

Overview

Unlike HTTP, WebSocket provides full-duplex communication.^[2][3] Additionally, WebSocket enables streams of messages on top of TCP. TCP alone deals with streams of bytes with no inherent concept of a message. Before WebSocket, port 80 full-duplex communication was attainable using Comet channels; however, Comet implementation is nontrivial, and due to the TCP handshake and HTTP header overhead, it is inefficient for small messages. WebSocket protocol aims to solve these problems without compromising security assumptions of the web.

The WebSocket protocol specification defines ws and wss as two new uniform resource identifier (URI) schemes^[4] that are used for unencrypted and encrypted connections, respectively. Apart from the scheme name and fragment (# is not supported), the rest of the URI components are defined to use URI generic syntax.^[5]

Using the browser Developer Tools, developers can inspect the WebSocket handshake as well as the WebSocket frames.^[6]

History

WebSocket was first referenced as TCPConnection in the HTML5 specification, as a placeholder for a TCP-based socket API.^[7] In June 2008, a series of discussions were led by Michael Carter that resulted in the first version of the protocol known as WebSocket.^[8]

The name Websocket was coined by Ian Hickson and Michael Carter shortly thereafter through collaboration on the #whatwg IRC chat room,^[9] and subsequently authored for inclusion in the HTML5 specification by Ian Hickson, and announced on the cometdaily blog by Michael Carter.^[10] In December 2009, Google Chrome 4 was the first browser to ship full support for the standard, with WebSocket enabled by default.^[11] Development of the WebSocket protocol was subsequently moved from the W3C and whatwg group to the IETF in February 2010, and authored for two revisions under Ian Hickson.^[12]

After the protocol was shipped and enabled by default in multiple browsers, the RFC was finalized under Ian Fette in December 2011.^[13]

Browser implementation

A secure version of the WebSocket protocol is implemented in Firefox 6,^[14] Safari 6, Google Chrome 14,^[15] Opera 12.10 and Internet Explorer 10.^[16] A detailed protocol test suite report^[17] lists the conformance of those browsers to specific protocol aspects.

An older, less secure version of the protocol was implemented in Opera 11 and Safari 5, as well as the mobile version of Safari in iOS 4.2.^[18] The BlackBerry Browser in OS7 implements WebSockets.^[19] Because of vulnerabilities, it was disabled in Firefox 4 and 5,^[20] and Opera 11.^[21]

There is also a command-line switch --enable-websocket-over-spdy for Google Chrome that enables an early experimental implementation of WebSocket over SPDY.^[22]

Protocol handshake

To establish a WebSocket connection, the client sends a WebSocket handshake request, for which the server returns a WebSocket handshake response, as shown in the example below.^[31]

Client request (just like in HTTP, each line ends with \r\n and there must be an extra blank line at the end):

GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: x3JJHMbDL1EzLkh9GBhXDw==
Sec-WebSocket-Protocol: chat, superchat
Sec-WebSocket-Version: 13
Origin: http://example.com

Server response:

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: HSmrc0sMlYUkAGmm5OPpG2HaGWk=
Sec-WebSocket-Protocol: chat

The handshake resembles HTTP in allowing servers to handle HTTP connections as well as WebSocket connections on the same port. Once the connection is established, communication switches to a bidirectional binary protocol which doesn't conform to the HTTP protocol.

In addition to Upgrade headers, the client sends a Sec-WebSocket-Key header containing base64-encoded random bytes, and the server replies with a hash of the key in the Sec-WebSocket-Accept header. This is intended to prevent a caching proxy from re-sending a previous WebSocket conversation,^[32] and does not provide any authentication, privacy or integrity. The hashing function appends the fixed string 258EAFA5-E914-47DA-95CA-C5AB0DC85B11 (a GUID) to the value from Sec-WebSocket-Key header (which is not decoded from base64), applies the SHA-1 hashing function, and encodes the result using base64.^[33]

Once the connection is established, the client and server can send WebSocket data or text frames back and forth in full-duplex mode. The data is minimally framed, with a small header followed by payload. WebSocket transmissions are described as "messages", where a single message can optionally be split across several data frames. This can allow for sending of messages where initial data is available but the complete length of the message is unknown (it sends one data frame after another until the end is reached and marked with the FIN bit). With extensions to the protocol, this can also be used for multiplexing several streams simultaneously (for instance to avoid monopolizing use of a socket for a single large payload).

It is important (from a security perspective) to validate the "Origin" header during the connection establishment process on the serverside (against the expected origins) to avoid Cross-Site WebSocket Hijacking attacks, which might be possible when the connection is authenticated with Cookies or HTTP authentication. It is better to use tokens or similar protection mechanisms to authenticate the WebSocket connection when sensitive (private) data is being transferred over the WebSocket.^[34]

Proxy traversal

WebSocket protocol client implementations try to detect if the user agent is configured to use a proxy when connecting to destination host and port and, if it is, uses HTTP CONNECT method to set up a persistent tunnel.

While the WebSocket protocol itself is unaware of proxy servers and firewalls, it features an HTTP-compatible handshake thus allowing HTTP servers to share their default HTTP and HTTPS ports (80 and 443) with a WebSocket gateway or server. The WebSocket protocol defines a ws:// and wss:// prefix to indicate a WebSocket and a WebSocket Secure connection, respectively. Both schemes use an HTTP upgrade mechanism to upgrade to the WebSocket protocol. Some proxy servers are transparent and work fine with WebSocket; others will prevent WebSocket from working correctly, causing the connection to fail. In some cases, additional proxy server configuration may be required, and certain proxy servers may need to be upgraded to support WebSocket.

If unencrypted WebSocket traffic flows through an explicit or a transparent proxy server without WebSockets support, the connection will likely fail.^[35]

If an encrypted WebSocket connection is used, then the use of Transport Layer Security (TLS) in the WebSocket Secure connection ensures that an HTTP CONNECT command is issued when the browser is configured to use an explicit proxy server. This sets up a tunnel, which provides low-level end-to-end TCP communication through the HTTP proxy, between the WebSocket Secure client and the WebSocket server. In the case of transparent proxy servers, the browser is unaware of the proxy server, so no HTTP CONNECT is sent. However, since the wire traffic is encrypted, intermediate transparent proxy servers may simply allow the encrypted traffic through, so there is a much better chance that the WebSocket connection will succeed if WebSocket Secure is used. Using encryption is not free of resource cost, but often provides the highest success rate since it would be travelling through a secure tunnel.

A mid-2010 draft (version hixie-76) broke compatibility with reverse proxies and gateways by including eight bytes of key data after the headers, but not advertising that data in a Content-Length: 8 header.^[36] This data was not forwarded by all intermediates, which could lead to protocol failure. More recent drafts (e.g., hybi-09^[37]) put the key data in a Sec-WebSocket-Key header, solving this problem.

See also

Notes

References

External links

• IETF Hypertext-Bidirectional (HyBi) working group
  • The WebSocket protocol - Proposed Standard published by the IETF HyBi Working Group
  • The WebSocket protocol - Internet-Draft published by the IETF HyBi Working Group
  • The WebSocket protocol - Original protocol proposal by Ian Hickson
• The WebSocket API - W3C Working Draft specification of the API
• The WebSocket API - W3C Candidate Recommendation specification of the API
• WebSocket.org WebSocket demos, loopback tests, general information and community